Plastic film with a multilayered interference coating

ABSTRACT

The invention relates to a plastic film with an interference multilayer system applied thereon that comprises at least two layers. Said layers can be obtained by hardening and/or applying thermal treatment to a coating composition containing nanoscale inorganic solid particles having polymerizable and/or polycondensable organic surface groups, thereby forming a layer that is crosslinked by means of the polymerizable and/or polycondensable organic surface groups. The films can be used as optical laminating film.

The invention relates to a polymer film with a multilayer interferencesystem applied to it, to a process for producing said film, to acomposite material comprising a substrate laminated with a polymer filmwith a multilayer interference system, and to uses of said film and ofsaid material.

Films of polymeric material which carry interference layer assemblies onone side are required, for example, for specialty filters or for certainoptical applications in architecture or in vehicle construction,particularly for specialty glazing, where they may be used asantireflection, NIR reflection, IR reflection or color filter layers.The polymer films with the interference layer assemblies are laminated,for example, to solid sheets of glass or plastic. Prior art interferencelayer assemblies comprising optical layers of high and low refractiveindex (λ/4 layers) are deposited by vacuum coating techniques(sputtering). With these techniques, however, the deposition rates whichcan be realized are low, and this is reflected in the high price of thefilms. Only purely inorganic layers can be applied by the sputteringtechnique.

Also known from the prior art are wet-chemical, sol-gel processcoatings. However, it has so far proven possible to apply these coatingsnot to flexible polymer films but only to rigid or solid glasssubstrates, such as flat glass and spectacle glass, or plasticssubstrates such as polycarbonate sheets. The rigid substrates have beencoated using dipping or spin coating techniques, which are unsuited tothe coating of flexible films.

Moreover, it is known that that flexible polymer films may be providedwith other functional coatings by wet-chemical methods for the purpose,for example, of producing magnetic tapes for audio or video cassettes,inkjet overhead films, or foils for surface decoration by means of hotstamping. This is done using film coating methods, examples being knifecoating (doctor blade coating), slot die coating, kiss coating withspiral scrapers, meniscus coating, roll coating or reverse-roll coating.

The production of multilayer optical interference systems films withthese wet-chemical coating techniques, however, is unknown.

It is an object of the invention to provide a simple process forproducing multilayer optical interference systems on polymer films, andcorresponding products, without the need for complicated and thus costlyvacuum coating techniques.

The invention provides a polymer film on which there has been applied amultilayer optical interference system comprising at least two layerseach obtainable by solidifying and/or heat-treating a coatingcomposition comprising nanoscale inorganic particulate solids havingpolymerizable and/or polycondensable organic surface groups to form alayer which is crosslinked by way of the polymerizable and/orpolycondensable organic surface groups. Each of the layers obtained isan organically modified inorganic layer.

The solidification can be brought about by heat treatment, lightexposure (UV, Vis), simple standing at room temperature, or acombination of these measures.

The invention further provides a process for producing this polymer filmwith multilayer interference coating, which comprises the followingsteps:

-   a) applying a coating sol comprising nanoscale inorganic particulate    solids having polymerizable and/or polycondensable organic surface    groups to the polymer film,-   b) solidifying the coating sol applied in a), where appropriate with    crosslinking of the polymerizable and/or polycondensable organic    surface groups of the particulate solids, to form an at least partly    organically crosslinked layer,-   c) applying a further coating sol comprising nanoscale inorganic    particulate solids having polymerizable and/or polycondensable    organic surface groups to the layer solidified in b),-   d) solidifying the coating sol applied in c), where appropriate with    crosslinking of the polymerizable and/or polycondensable organic    surface groups of the particulate solids, to form a further    solidified layer,-   e) if desired, repeating steps c) and d) one or more times to form    solidified layers, and-   f) heat-treating and/or irradiating the resultant layer assembly, it    being possible to perform this step together with step d) for the    topmost layer.

Steps b) and d) are carried out by heat treatment, irradiation (UV,Vis), simple standing at room temperature, or a combination of thesemethods.

In step f) a heat treatment is preferable.

A multilayer interference system is composed of at least two layers ofmaterials having different refractive indices. A fraction of theincident light is reflected at each of the interfaces between thelayers. Depending on the material and the thickness of the layers, thereflections are extinguished (negative interference) or intensified(positive interference).

Surprisingly it has been found that with the coating composition used inaccordance with the invention it is possible to provide polymer filmswith a multilayer interference system in a wet-chemical film coatingprocess. In accordance with the invention the desired refractive indicesfor each layer can be set in a targeted way by selection of the coatingcompositions, with the at least two layers having different refractiveindices.

In the present description “nanoscale inorganic particulate solids” arein particular those having an average particle diameter of not more than200 nm, preferably not more than 100 nm, and in particular not more than70 nm, e.g., from 5 to 100 nm, preferably from 5 to 70 nm. Oneparticularly preferred particle size range is from 5 to 10 nm.

The nanoscale inorganic particulate solids may be composed of anydesired materials but are preferably composed of metals and inparticular metal compounds such as (optionally hydrated) oxides such asZnO, CdO, SiO₂, TiO₂, ZrO₂, CeO₂, SnO₂, Al₂O₃, In₂O₃, La₂O₃, Fe₂O₃,Cu₂O, Ta₂O₅, Nb₂O₅, V₂O₅, MoO₃ or WO₃, chalcogenides such as sulfides(e.g., CdS, ZnS, PbS, an Ag₂S), selenides (e.g., GaSe, CdSe, and ZnSe),and tellurides (e.g., ZnTe or CdTe), halides such as AgCl, AgBr, Agl,CuCl, CuBr, Cdl₂, and Pbl₂; carbides such as CdC₂ or SiC; arsenides suchas AlAs, GaAs, and, GeAs; antimonides such as InSb; nitrides such as BN,AlN, Si₃N₄, and Ti₃N₄; phosphides such as GaP, InP, Zn₃P₂, and Cd₃P₂;phosphates, silicates, zirconates, aluminates, stannates, and thecorresponding mixed oxides (e.g., indium-tin oxides (ITO) and those withperovskite structure such as BaTiO₃ and PbTiO₃).

The nanoscale inorganic particulate solids used in the process of theinvention are preferably (optionally hydrogenated) oxides, sulfides,selenides, and tellurides of metals and mixtures thereof. Preferred inaccordance with the invention are nanoscale particles of SiO₂, TiO₂,ZrO₂, ZnO, Ta₂O₅, SnO₂, and Al₂O₃ (in all modifications, especially asboehmite, AlO(OH)), and mixtures thereof. It has proven to be the casethat SiO₂ and/or TiO₂ as nanoscale inorganic particulate solids for theparticulate solids having polymerizable and/or polycondensable organicsurface groups produce coating compositions particularly suitable forfilm coating. The nanoscale particles still contain reactive groups onthe surfaces; for example, on the surfaces of oxide particles there aregenerally hydroxide groups.

Since the nanoscale particles which can be used in accordance with theinvention span a broad range of refractive indices, appropriateselection of these nanoscale particles allows the refractive index ofthe layer(s) to be set easily to the desired value.

The nanoscale particulate solids used in accordance with the inventionmay be produced conventionally: for example, by flame pyrolysis, plasmaprocesses, gas-phase condensation processes, colloid techniques,precipitation processes, sol-gel processes, controlled nucleation andgrowth processes, MOCVD processes, and (micro)emulsion processes. Theseprocesses are described in detail in the literature. It is possible inparticular to draw, for example, on metals (for example, after thereduction of the precipitation processes), ceramic oxidic systems (byprecipitation from solution), an also saltlike or multicomponentsystems. The saltlike or multicomponent systems also includesemiconductor systems.

Use may also be made of commercially available nanoscale inorganicparticulate solids. Examples of commercially available nanoscale SiO₂particles are commercial silica products, e.g., silica sols, such as theLevasils®, silica sols from Bayer AG, ®Klebosol from Clariant, or fumedsilicas, e.g., the Aerosil products from Degussa.

The preparation of the nanoscale inorganic particulate solids providedwith polymerizable and/or polycondensable organic surface groups thatare used in accordance with the invention may in principle be carriedout in two different ways, namely first by surface modification ofpre-prepared nanoscale inorganic particulate solids and secondly bypreparation of these inorganic nanoscale particulate solids using one ormore compounds which possess such polymerizable and/or polycondensablegroups. These two ways are elucidated later on below and in theexamples.

The organic polymerizable and/or polycondensable surface groups maycomprise any groups known to the skilled worker that are amenable tofree-radical, cationic or anionic, thermal or photochemicalpolymerization or to thermal or photochemical polycondensation, with oneor more suitable initiators and/or catalysts possibly being present. Theexpression “polymerization” here also includes polyaddition. Theinitiators and/or catalysts which may be used where appropriate for therespective groups are known to the skilled worker. In accordance withthe invention preference is given to surface groups which possess a(meth)acryloyl, allyl, vinyl or epoxy group, with (meth)acryloyl andepoxy groups being particularly preferred. The polycondensable groupsinclude in particular hydroxyl, carboxyl, and amino groups, by means ofwhich ether, ester, and amide linkages can be obtained between thenanoscale particles.

As already mentioned, the polymerizable and/or polycondensable surfacegroups may in principle be provided in two ways. Where surfacemodification of pre-prepared nanoscale particles is carried out,compounds suitable for this purpose are all those (preferably of lowmolecular mass) which on the one hand possess one or more groups whichare able to react or at least interact with reactive groups present onthe surface of the nanoscale particulate solids (such as OH groups, forexample, in the case of oxides) and on the other hand contain at leastone polymerizable and/or polycondensable group. Surface modification ofthe nanoscale particles may be accomplished, for example, by mixing themwith suitable compounds elucidated below, where appropriate in a solventand in the presence of a catalyst. Where the surface modifiers aresilanes, it is sufficient, for example, to stir them with the nanoscaleparticles at room temperature for a number of hours.

Accordingly, the corresponding compounds may, for example, form not onlycovalent but also ionic (saltlike) or coordinative (complex) bonds tothe surface of the nanoscale particulate solids, whereas simpleinteractions include, for example, dipole-dipole interactions, hydrogenbonding, and van der Waals interactions. Preference is given to theformation of covalent and/or coordinate bonds.

In accordance with the invention it is also preferred for the organicgroups which are present on the surfaces of the nanoscale particles andwhich include the polymerizable and/or polycondensable groups to have arelatively low molecular weight. In particular, the molecular weight ofthe (purely organic) groups should not exceed 500 g/mol and preferably300 g/mol, more preferably 200 g/mol. This does not of course rule outthe compounds (molecules) containing these groups having a significantlyhigh molecular weight (e.g., up to 1000 g/mol or more).

Examples of organic compounds which can be used to modify the surfacesof the nanoscale inorganic particulate solids include unsaturatedcarboxylic acids, β-dicarbonyl compounds, e.g., β-diketones orβ-carbonylcarboxylic acids, having polymerizable double bonds,ethylenically unsaturated alcohols and amines, amino acids, and epoxidesand diepoxides. Compounds used with preference for surface modificationare diepoxides, β-diketones, methacryloylsilanes, and epoxysilanes.

Specific examples of organic compounds for surface modification arediepoxides such as 3,4-epoxycyclohexylmethyl3,4-epoxycyclohexanecarboxylate, bis(3,4-epoxycyclohexyl) adipate,cyclohexanedimethanol diglycidyl ether, neopentylglycol diglycidylether, 1,6-hexanediol diglycidyl ether, propylene glycol diglycidylether, bisphenol A diglycidyl ether, bisphenol F diglycidyl ether, andunsaturated carboxylic acids such as acrylic acid and methacrylic acid.

Further particularly preferred compounds for surface modification of thenanoscale inorganic particulate solids are in particular, in the case ofoxidic particles, hydrolytically condensable silanes having at least(and preferably) one nonhydrolyzable radical which possesses anpolymerizable and/or polycondensable group, this being preferably apolymerizable carbon-carbon double bond or an epoxy group. Silanes ofthis kind preferably have the formula (I):X—R¹—SiR² ₃  (I)in which X is CH₂═CR³—COO, CH₂═CH, epoxy, glycidyl or glycidyloxy, R³ ishydrogen or methyl, R¹ is a divalent hydrocarbon radical having 1 to 10,preferably 1 to 6, carbon atoms, which if desired contains one or moreheteroatom groups (e.g., O, S, NH), which separate adjacent carbon atomsfrom one another, and the radicals R², identical to or different fromone another, are selected from alkoxy, aryloxy, acyloxy, andalkylcarbonyl groups and also halogen atoms (especially F, Cl and/orBr).

The groups R² may be different from one another but are preferablyidentical. The groups R² are preferably selected from halogen atoms,C₁₋₄ alkoxy groups (e.g., methoxy, ethoxy, n-propoxy, isopropoxy andbutoxy), C₆-C₁₀ aryloxy groups (e.g., phenoxy), C₁₋₄ acyloxy groups(e.g., acetoxy and propionyloxy), and C₂₋₁₀ alkylcarbonyl groups (e.g.,acetyl). Particularly preferred radicals R² are C₁₋₄ alkoxy groups andespecially methoxy and ethoxy.

The radical R¹ is preferably an alkylene group, especially one having 1to 6 carbon atoms, such as methylene, ethylene, propylene, butylene, andhexylene, for example. If X is CH₂═CH, R¹ is preferably methylene and inthis case may also simply be a bond.

X is preferably C₂═CR³—COO (where R³ is preferably CH₃) or glycidyloxy.Accordingly, particularly preferred silanes of the formula (I) are(meth)acryloxyalkyltrialkoxysilanes, such as3-methacryloxypropyltrimethoxysilane and3-methacryloxypropyltriethoxysilane, for example, andglycidyloxyalkyltrialkoxysilanes, such as3-glycidyloxypropyltrimethoxysilane and3-glycidyloxypropyltriethbxysilane, for example.

Where the nanoscale inorganic particulate solids are actually preparedusing one or more compounds which possess polymerizable and/orpolycondensable groups it is possible to forego subsequent surfacemodification, although such modification is of course possible as anadditional measure.

The in situ preparation of nanoscale inorganic particulate solids havingpolymerizable and/or polycondensable surface groups is elucidated below,taking SiO₂ particles as example. For this purpose the SiO₂ particles,for example, can be prepared by the sol-gel process using at least onehydrolytically polycondensable silane having at least one polymerizableand/or polycondensable group. Suitable such silanes include, forexample, the above-described silanes of the formula (I). These silanesare used either alone or in combination with a suitable silane of theformula (II)SiR² ₄  (II)in which R² is as defined above. Preferred silanes of the above formula(II) are tetramethoxysilane and tetraethoxysilane.

In addition to or alternatively to the silanes of the formula (II) it isof course also possible to use other hydrolyzable silanes, examplesbeing those which possess at least one nonhydrolyzable hydrocarbon groupwithout a polymerizable and/or polycondensable group, such as methyl- orphenyltrialkoxysilanes, for example. These may be silanes of the formula(III)R⁴ _(n)SiR² _(4-n)  (III)in which R² is as defined above and the nonhydrolyzable radical R⁴ is analkyl group, preferably C₁₋₆ alkyl, such as methyl, ethyl, n-propyl,isopropyl, n-butyl, s-butyl and t-butyl, pentyl or hexyl, for example, acycloalkyl group having 5 to 12 carbon atoms, such as cyclohexyl, or anaryl group, preferably C₆₋₁₀ aryl, such as phenyl or naphthyl, forexample, and n is 1, 2 or 3, preferably 1 or 2, and especially 1. Saidradical R⁴ may if desired have one or more customary substituents, suchas halogen, ether, phosphoric acid, sulfonic acid, cyano, amido,mercapto, thioether or alkoxy groups, for example.

In the process of the invention a coating composition comprising theaforementioned nanoscale inorganic particulate solids is applied to thepolymer film or to a layer which has already been applied. The appliedcoating composition is in particular a coating sol, i.e., a dispersionof the above-defined nanoscale inorganic particulate solids havingpolymerizable and/or polycondensable organic surface groups in a solventor solvent mixture. The coating composition is fluid on application.

The solvent may be any solvent known to the skilled worker. The solventmay be, for example, water and/or an organic solvent. The organicsolvent is preferably miscible with water. Examples of suitable organicsolvents are alcohols, ethers, ketones, esters, amides, and mixturesthereof. Preference is given to using alcohols, e.g., aliphatic oralicyclic alcohols, or alcohol mixtures, as solvent, preference beinggiven to monohydric alcohols. Preferred alcohols are linear or branchedmonovalent alkanols having 1 to 8, preferably 1 to 6, carbon atoms.Particularly preferred alcohols are methanol, ethanol, n-propanol,2-propanol, n-butanol, 2-butanol, isobutanol, tert-butanol or mixturesthereof.

The solvent or part of it may also be formed during the preparation ofthe nanoscale particles or during the surface modification. For example,the preparation of SiO₂ particles from alkoxysilanes is accompanied byliberation of the corresponding alcohols, which can then act assolvents.

It has surprisingly been found that the coating sols for the coating inaccordance with the invention are especially suitable when they are usedin highly diluted form for application. The total solids content of thecoating sol to be applied is advantageously not more than 40% by weight,preferably not more than 20% by weight, in particular not more than 15%by weight. The total solids content is preferably from 0.5 to 5% byweight, more preferably from 1 to 3% by weight.

Provided the total solids content of the coating sol is not more than40% by weight, excellent coatings can be obtained on the polymer films.If the coating sol is in a low dilution, i.e., has a relatively hightotal solids content, high wet film thicknesses of the coating cannot beachieved. Suitable wet film thicknesses of the applied coating sol aresituated typically in the lower μm range, e.g., from 0.5 μm to 10 μm.

An additional constituent of the coating sol may be, for example, atleast one monomeric or oligomeric species possessing at least one groupwhich is able to react (undergo polymerization and/or polycondensation)with the polymerizable and/or polycondensable groups present on thesurface of the nanoscale particles. Suitable such species include, forexample, monomers having a polymerizable double bond, such as acrylates,methacrylates, styrene, vinyl acetate, and vinyl chloride, for example.Preferred monomeric compounds having more than one polymerizable bondare, in particular, those of the formula (IV):(CH₂═CR³—COZ-)_(m)-A  (IV)in which

-   m=2, 3 or 4, preferably 2 or 3, and especially 2,-   Z=O or NH, preferably O,-   R³═H, CH₃,-   A=m-valent hydrocarbon radical which has 2 to 30, especially 2 to    20, carbon atoms and can contain one or more heteroatom groups    located in each case between two adjacent carbon atoms (examples of    such heteroatom groups are O, S, NH, NR(R=hydrocarbon radical),    preferably O).

The hydrocarbon radical A may also carry one or more substituentsselected preferably from halogen (especially F, Cl and/or Br), alkoxy(especially C₁₋₄ alkoxy), hydroxyl, unsubstituted or substituted amino,NO₂, OCOR⁵, COR⁵ (R⁵═C₁₋₆ alkyl or phenyl). Preferably, however, theradical A is unsubstituted or is substituted by halogen and/or hydroxyl.

In one embodiment of the present invention A is derived from analiphatic diol, an alkylene glycol, a polyalkylene glycol, or anoptionally alkoxylated (e.g., ethoxylated) bisphenol (e.g., bisphenolA).

Further useful compounds having more than one double bond are, forexample, allyl (meth)acrylate, divinylbenzene, and diallyl phthalate.Similarly, for example, a compound having two or more epoxy groups canbe used (in the case where epoxide-containing surface groups are used),e.g., bisphenol A diglycidyl ether, or else an (oligomeric)precondensate of an epoxy-functional hydrolyzable silane such asglycidoxypropyltrimethoxysilane.

The fraction of organic components in the coating sols used inaccordance with the invention is preferably not more than 20% by weight,e.g., from 4 to 15% by weight, based on the total solids content. Forlayers of high refractive index, for example, it can be 5% by weight,for layers with low refractive index, for example, 15% by weight.Preferably, however, no such organic components are used.

The coating sol used in accordance with the invention preferably has apH≧3, more preferably ≧4. Generally speaking the pH is situated withinthe neutral range up to about 8, preferably up to about 7.5.

If desired, further additives, customary for film coatings, may also beadded to the coating sol. Examples are thermal or photochemicalcrosslinking initiators, sensitizers, wetting auxiliaries, adhesionpromoters, leveling agents, antioxidants, stabilizers, crosslinkingagents, metal colloids, e.g., as carriers of optical functions. Besidesthe thermal or photochemical crosslinking initiators which may be used,and which are elucidated later on, however, the coating sol contains nofurther components; in other words, the coating sol or coatingcomposition consists preferably of the nanoscale particulate solidshaving polymerizable and/or polycondensable organic surface groups, thesolvent or solvents, and, if desired, one or more thermal orphotochemical crosslinking initiators.

Unlike other rigid substrates, films are flexible and therefore requirespecial coating techniques and coating compositions. The polymer film tobe coated may be a conventional film customary in the art, preferably afilm of limited length. Specific examples are films of polyethylene,e.g., HDPE or LDPE, polypropylene, cellulose triacetate (TAC),polyisobutylene, polystyrene, polyvinyl chloride, polyvinylidenechloride, polytetrafluoroethylene, polychlorotrifluoroethylene,polyamide, poly(meth)acrylates, polyethylene terephthalate,polycarbonate, regenerated cellulose, cellulose nitrate, celluloseacetate, cellulose triacetate, cellulose acetate butyrate or rubberhydrochloride. The polymer film is preferably transparent. It is ofcourse also possible to use composite films formed, for example, fromthe materials mentioned above.

The polymer film may have been pretreated. Prior to coating inaccordance with the invention it may undergo, for example, a coronatreatment or may be provided with a precoat, in order for example topromote adhesion, with a hard coat and/or with an antiglare coating.

In step a) of the process of the invention the coating sol is applied tothe polymer film by a film coating process in order to coat all or partof said film (on one side). Coating takes place on individual filmsections or, preferably, in a continuous coating process. Suitablecoating processes are the conventional film coating processes known tothe skilled worker. Examples thereof are knife coating (doctor bladecoating processes), slot die coating, kiss coating with spiral scrapers,meniscus coating, roll coating or reverse-roll coating.

For the process of the invention, reverse-roll coating has provenparticularly appropriate. In this process, the coating sol is taken upby a dip roll and transferred via a meniscus coating step, using atransfer roll, to a pressure roll (master roll). Given appropriatelyhigh precision of the rolls and of the drives, the nip between two rollsis sufficiently constant. The wet film present on the pressure roll isthen deposited, usually completely, on the substrate film. As a result,the thickness of the wet film deposite on the substrate film isindependent of any fluctuations in the thickness of the substrate film.Through the use of the reverse-roll process it is possible,surprisingly, to apply particularly precise and uniform multilayerinterference systems to polymer films, and so the use of this processconstitutes one particularly preferred embodiment of the process of theinvention.

Before being applied to this film, the coating sol can be adjusted to asuitable viscosity or to a suitable solids content by means, forexample, of addition of solvent. This involves preparation in particularof the highly diluted coating sols set out above. Application may befollowed by a drying step, particularly when crosslinking is not carriedout by a heat treatment.

In step b) of the process of the invention the coating sol applied in a)is solidified, by evaporating the solvent and/or by crosslinking thepolymerizable and/or polycondensable surface groups of the nanoscaleinorganic particulate solids, for example (where appropriate by way ofthe polymerizable and/or polycondensable groups of the monomeric oroligomeric species additionally used). Crosslinking may be carried outby means of customary polymerization and/or polycondensation reactionsin the manner familiar to the skilled worker.

Examples of suitable crosslinking methods are thermal and photochemical(e.g., UV) crosslinking, electron beam curing, laser curing orroom-temperature curing. Crosslinking takes place where appropriate inthe presence of a suitable catalyst or initiator, which is added to thecoating sol no later than immediately before application to the film.

Suitable initiators/initiator systems include all commonplaceinitiators/initiator systems known to the skilled worker, includingfree-radical photoinitiators, free-radical thermoinitiators, cationicphotoinitiators, cationic thermoinitiators, and any desired combinationsthereof.

Specific examples of free-radical photoinitiators which can be usedinclude Irgacure® 184 (1-hydroxycyclohexyl phenyl ketone), Irgacure® 500(1-hydroxycyclohexyl phenyl ketone, benzophenone), and other Irgacure®photoinitiators available from Ciba-Geigy; Darocur 1173, 1116, 1398,1174, and 1020 (available from Merck); benzophenone,2-chlorothioxanthone, 2-methylthioxanthone, 2-isopropyl-thioxanthone,benzoin, 4,4′-dimethoxybenzoin, benzoin ethyl ether, benzoin isopropylether, benzil dimethyl ketal, 1,1,1-trichloroacetophenone,diethoxyacetophenone, and dibenzosuberone.

Examples of free-radical thermoinitiators include organic peroxides inthe form of diacyl peroxides, peroxydicarbonates, alkyl peresters, alkylperoxides, perketals, ketone peroxides, and alkyl hydroperoxides, andalso azo compounds. Specific examples that might be mentioned hereinclude, in particular, dibenzoyl peroxide, tert-butyl perbenzoate, andazobisisobutyronitrile. Where epoxy groups are present for thecrosslinking it is possible to use, as thermoinitiators, compoundscontaining amine groups. An example is an aminopropyltrimethoxysilane.

One example of a cationic photoinitiator is Cyracure® UVI-6974, while apreferred cationic thermoinitiator is 1-methylimidazole.

These initiators may be used in the normal amounts known to the skilledworker, e.g., from 0.01 to 5% by weight, in particular from 0.1 to 2% byweight, based on the total solids content of the coating sol. In somecases it is of course possible in certain circumstances to do withoutthe initiator entirely.

The crosslinking in step b) of the process of the invention takes placepreferably thermally or by irradiation (in particular with UV light).Conventional light sources can be used for photopolymerization,especially sources which emit UV light, e.g., mercury vapor lamps, xenonlamps and laser light. In the case of crosslinking by way of heattreatment the appropriate temperature range depends naturally inparticular on the extant polymerizable and/or polycondensable surfacegroups of the nanoscale inorganic particulate solids, on any initiatorsused, on the degree of dilution, and the duration of the treatment.

Generally speaking, heat treatment for crosslinking in b) and d) takesplace within a temperature range from 20 to 130° C., preferably from 80to 120° C., in particular from 100 to 120° C. The duration of thetreatment may be, for example, from 30 seconds to 5 minutes, preferablyfrom 1 to 2 minutes. Steps b) and d) are performed such that at leastpartial crosslinking has taken place by way of the polymerizable and/orpolycondensable surface groups; it is also possible for substantiallyall of the polymerizable and/or polycondensable surface groups to beconsumed by reaction for the crosslinking in this step.

In the course of heat treatment, further volatile constituents,especially the solvent, may evaporate from the coating compositionbefore, during or after crosslinking, generally at the same time ascrosslinking. Where no heat treatment is carried out for crosslinking, aheat treatment (for drying) may be performed following crosslinking.

The rate at which the coating composition is applied is chosen, as afunction of the desired refractive index and field of application,generally in such a way as to obtain dry film thicknesses in the rangefrom 50 to 300 nm, preferably from 100 to 150 nm.

In accordance with steps c) and d) and, where appropriate, e), one ormore further layers are applied to the solidified layer formed, inanalogy to steps a) and b), until the desired assembly of layers isobtained. In the case of the last (topmost) layer there is no longerabsolute need for a separate crosslinking step as per b) and/or d);instead, crosslinking can be carried out, if desired, directly togetherwith the final heat treatment step f) for aftertreating the layerassembly.

In step f), the layer assembly undergoes heat treatment. The heattreatment depends naturally on the film and on the composition of thelayers. Generally speaking, however, the final heat treatment takesplace at temperatures in the range from 20 to 200° C., preferably from80 to 200° C., more preferably from 100 to 160° C., and in particularfrom 110 to 130° C. The duration of the heat treatment is, for example,from 10 minutes to 24 h, preferably from 3 minutes to 1 h. This givesmultilayer interference systems on polymer film without cracking orother defects.

Within the layers and/or within the layer assembly, the final heattreatment of the layer assembly may lead, for example, to substantialcompletion of the organic crosslinking or, where appropriate, may removeany residues of solvent present. During the heat treatment there mayalso be condensation reactions between the reactive groups still presenton the surface of the particulate solids (e.g., (Si)—OH groups on SiO₂particles), so that the solids particles within the layers are linked toone another by inorganic condensation reactions as well as the organiccrosslinking elucidated above.

In one particularly preferred embodiment it is possible to select thecoating compositions such that, in the finished polymer film withmultilayer interference system, residual reflection values of below 0.5%in the range between 400 nm and 650 nm wavelength, and residualreflection values below 0.3% at a wavelength of 550 nm, are obtained.

Depending on the end application, further treatment steps may follow. Onthe side opposite the side bearing the multilayer system, for example,the coated film may be provided with an adhesive layer and, whereappropriate, with a top layer. The adhesive layer may serve, forexample, for lamination to a substrate. A continuous film can be cut tosize in order to obtain dimensions suitable for the end application.

The polymer film of the invention with multilayer interference systemapplied thereto is particularly suitable as an optical laminating filmfor glass and plastics substrates. Accordingly, the invention alsoprovides a composite material comprising a film, a paint layer or asubstrate, in particular a rigid substrate, which is preferably composedof glass or plastic and is preferably transparent, onto which thepolymer film of the invention is laminated.

Suitable laminating techniques are known to the skilled worker, and anycustomary laminating techniques can be employed. Joining takes place,for example, by way of an adhesive layer, which may be applied to thefilm, to the substrate, or to both. Where needed, an interference layermay also be applied to the reverse side of the substrate.

The polymer films with multilayer interference system produced inaccordance with the invention, or the corresponding composite materials,are suitable, for example, as antireflection systems, especially toreduce glare, as reflection systems, reflection filters, and colorfilters for lighting purposes or for decorative purposes.

Examples of specific applications for the polymer films with multilayerinterference system produced in accordance with the invention, and thecorresponding composite materials, include the following:

-   -   antireflection systems and antireflection coatings for visible        light in the field of architecture, e.g., screens or windows in        buildings, glazing for shop windows and pictures, for        glasshouses, for glazing within vehicles, e.g., automobiles,        trucks, motorbikes, boats, and aircraft; or for instruments with        a display element, e.g., computer screens, television screens,        or cellphone displays;    -   coated films in roll form with optical and/or decorative effect;    -   NIR (near infrared) reflection filters;    -   antiglare coatings (NIR, Vis), e.g., for photovoltaic and other        optical applications (solar cells, solar collectors);    -   color filters for lighting or for decorative purposes;    -   IR reflection coat for fire protection and heat protection        applications;    -   optical films, such as polarization films, retarder films;    -   effect coating on painted surfaces;    -   UV reflection films; and    -   laser mirrors.

EXAMPLE Production of a Triple Antireflection Coat

1 Synthesis of the Coating Sols

The antireflection coating sols M, H, and L (M: sol for layer withmedium refractive index, H: sol for layer with high refractive index, L:sol for layer with low refractive index) were prepared from three basesols (H, Lr, and Lo).

1.1. Synthesis of the Base Sols (Room Temperature)

a) Base sol H

12.12 g of HCl (16.9% strength by weight) were added to a mixture of 400g of 2-propanol and 400 g of 1-butanol. 79.61 g of titanium isopropoxidewere added with stirring to the solvent mixture. Synthesis is completeafter stirring for 24 hours.

b) Base Sol Lr

105.15 g of tetraethoxysilane were dissolved in 60 g of ethanol.Additionally, a solution was prepared from 41.5 g of HCl (0.69% strengthby weight) and 60 g of ethanol and was added with stirring to thetetraethoxysilane/ethanol mixture. After a reaction time of 2 hours thesol was diluted with 500 g of 2-propanol and 500 g of 1-butanol.

c) Base Sol Lo

360.8 g of tetramethoxysilane were dissolved in 319.2 g of ethanol.Additionally, a solution was prepared from 6.9 g of HCl (37% strength byweight), 362.5 g of water and 319.2 g of butanol. This solution wasadded with stirring to the tetramethoxysilane/ethanol mixture. Synthesisis complete after stirring for 2 hours.

1.2. Preparation the Coating Sols (about 1 l of Sol)

a) Sol M

76.8 g of base sol Lr were mixed with 419.2 g of base sol H. 2.976 g of1,4-cyclohexanedimethanol diglycidyl ether (CHMG) were added dropwisewith stirring to this mixture. The sol was diluted with 321.6 g of1-butanol.

b) Sol H

1.2 g of 1,4-cyclohexanedimethanol diglycidyl ether (CHMO) were addeddropwise with stirring to 480 g of base sol H. The sol was diluted with321.6 g of 1-butanol.

c) Sol L

201.6 g of base sol Lo were diluted with 624 g of 1-butanol. 1.44 g ofprehydrolyzed glycidyloxypropyltrimethoxysilane (hydrolysis with 0.1 NHCl (0.5 mol/mol OR) were added to this mixture, and then the solventwas removed on a rotary evaporator. Additionally, 0.072 g ofaminopropyltrimethoxysliane, as thermoinitiator, was added to thismixture.

2. Coating of the Polymer Film

The polymer film used was a TAC film having a thickness of 50 μm and ahardcoat. The above coating sols M, H, and L were applied to the polymerfilm in succession with the aid of a reverse-roll coating unit (model BA12300, Werner Mathis AG, Switzerland). The film tension for all 3coatings is 60 N. Initial crosslinking of all three applied coatings wascarried out at an oven temperature of 120° C. for a period of 2 minutes.For aftertreatment the applied layer assembly in roll form was treatedin a preheated oven at 120° C. for 30 minutes, then removed and cooledto room temperature. The result was a flawless multilayer interferencesystem on the polymer film, having the desired interference behavior.

For the application of the 3 layers the following coating parameterswere set for the reverse-roll coating: Speed Sol Roller Rotation (m/min)Nip (μm) M Dip roller left 1.0 Transfer roller left 1.0 Between dip andtransfer roll = 100 Master roller right 1.0 Between dip and transferroll = 100 H Dip roller left 1.0 Transfer roller left 1.0 Between dipand transfer roll = 150 Master roller right 1.0 Between dip and transferroll = 100 L Dip roller left 1.0 Transfer roller left 1.0 Between dipand transfer roll = 100 Master roller right 1.0 Between dip and transferroll = 100

1) A polymer film on which there has been applied a multilayer opticalinterference system comprising at least two layers each obtainable bysolidifying and/or heat-treating a coating composition comprisingnanoscale inorganic particulate solids having polymerizable and/orpolycondensable organic surface groups to form a layer which iscrosslinked by way of the polymerizable and/or polycondensable organicsurface groups. 2) The polymer film as claimed in claim 1, wherein theat least two layers have different refractive indices. 3) The polymerfilm as claimed in claim 1, wherein the coating composition is a sol. 4)The polymer film as claimed in claim 1, wherein the nanoscaleparticulate solids are selected from the group consisting of SiO₂, TiO₂,ZrO₂ Ta₂O₅ and mixtures thereof. 5) The polymer film as claimed in claim1, wherein the polymerizable and/or polycondensable surface groups areselected from organic radicals which possess an acyl, methacryloyl,vinyl, allyl, or epoxy group. 6) A composite material comprising asubstrate onto which the polymer film as claimed in claim 1 islaminated. 7) A process for producing a polymer film with a multilayerinterference system, as claimed in claim 1, which comprises thefollowing steps: a) applying a coating sol comprising nanoscaleinorganic particulate solids having polymerizable and/or polycondensableorganic surface groups to the polymer film, b) solidifying the coatingsol applied in step a), where appropriate with crosslinking of thepolymerizable and/or polycondensable organic surface groups of theparticulate solids, to form an at least partly organically crosslinkedlayer, c) applying a further coating sol comprising nanoscale inorganicparticulate solids having polymerizable and/or polycondensable organicsurface group to the layer solidified in step b), d) solidifying thecoating sol applied in step c), where appropriate with crosslinking ofthe polymerizable and/or polycondensable organic surface groups of theparticulate solids, to form a further solidified layer, e) if desired,repeating steps c) and d) one or more times to form solidified layers,and f) heat-treating and/or irradiating the resultant layer assembly, itbeing possible to perform this step together with step d) for thetopmost layer. 8) The process as claimed in claim 7, wherein a coatingsol having a total solids content of not more than 40% by weight isapplied. 9) The process as claimed in claim 7, wherein the heattreatment step f) of the layer assembly is conducted at temperatures inthe range from 20 to 200° C. 10) The process as claimed in claim 7 to 9,wherein the coatings are applied by reverse-roll coating. 11) Anantireflection system, reflection system, reflection filter, colorfilter, light intensifier, polarization filter, retarder film or effectcoating on painted surfaces which uses the polymer film as claimed inclaim 1 or the composite material as claimed in claim
 6. 12) Theantireflection system, reflection filter, color filter or lightintensifier of claim 11 which is used in computer screens, displayglasses and lenses of cellphones, architectural glass and automobilewindow glass. 13) The process as claimed in claim 7, wherein the heattreatment step f) of the layer assembly is conducted at temperatures inthe range from 80 to 200° C.